Improved Methodology for Harmonics Reduction using Shunt Active ...

International Journal of Science and Research (IJSR) ISSN (Online): 2319-7064

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Volume 4 Issue 6, June 2015

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Improved Methodology for Harmonics Reduction

using Shunt Active Power Filter Based on p-q

Theory

Priyanka Zamade1, Minal Tomar

2

1PG research scholar, Malwa Institute of Technology, Indore, India

2Professor, Assistant Professor, Malwa Institute of Technology, Indore, India

Abstract: This paper discuss about the problem of harmonics occurring in various power electronic equipments. It proposes a simple

method for enhancement of power quality using concept of Shunt Active Power Filter (SAPF). In this paper SAPF is modeled using p-q

theory with PI control method so that Total Harmonic Distortion (THD) should be in compliance with IEEE 519 standard. The

compensation characteristics of each topology with the respective control scheme are proved by using MATLAB/SIMULINK.

Keywords: Harmonics Compensation, SAPF, p-q theory, THD, PI controller

1. Introduction

Power quality determines the fitness of electrical power to

consumer devices. Synchronization of the voltage frequency

and phase allows electrical systems to function in their

intended manner without significant loss of performance or

life. The term is used to describe electric power that drives

an electrical load and the load's ability to function properly.

Without the proper power, an electrical device (or load) may

malfunction, fail prematurely or not operate at all. There are

many ways in which electric power can be of poor quality

and many more causes of such poor quality power. Current

harmonics produced by non-linear loads, such as switching

power supplies and motor speed controllers, are prevalent in

today’s power systems. These harmonics interfere with

sensitive electronic equipment and cause unnecessary losses

in electrical equipment. Harmonics voltages and currents in

an electric power system are a result of non-linear electric

loads. Harmonic frequencies in the power grid are a frequent

cause of power quality problems. Harmonics in power

systems result in increased heating in the equipment and

conductors, misfiring in variable speed drives, and torque

pulsations in motors. Reduction of harmonics is considered

desirable. A harmonic of a wave is a component frequency

of the signal that is an integer multiple of the fundamental

frequency, i.e. if the fundamental frequency is f, the

harmonics have frequencies 2f, 3f, 4f . . . etc. The harmonics

have the property that they are all periodic at the

fundamental frequency; therefore the sum of harmonics is

also periodic at that frequency. Harmonic frequencies are

equally spaced by the width of the fundamental frequency

and can be found by repeatedly adding that frequency.

Figure 1: Difference between Linear and Non-Linear Loads

The terms “linear” and “non-linear” define the relationship

of current to the voltage waveform. A linear relationship

exists between the voltage and current, which is typical of an

across-the-line load. A non-linear load has a discontinuous

current relationship that does not correspond to the applied

voltage waveform.

2. Various Methodologies For Harmonic

Mitigation

The presence of harmonics in the power system cause greater

power loss in distribution, interference problem in

communication system and, sometimes results in operation

failure of electronic equipment which are more and more

sensitive. In order to reduce this problem various types of

filters are used using different methods which are as follows:

A. Passive Filters Passive implementations of linear filters are based on

combinations of resistors (R), inductors (L) and capacitors

(C). These types are collectively known as passive filters,

because they do not depend upon an external power supply

and/or they do not contain active components such as

transistors. Passive filters are used to mitigate power quality

problems in six pulse ac-dc converter. Apart from mitigating

the current harmonics, passive filters also provide reactive

power compensation, thereby further improving the system

performance. Passive filters have been used as a solution to

solve harmonic current problems, but because of the several

disadvantage of passive filter like it can mitigate only few

harmonics, gives rise to resonance problem, bulky in size

and costly they are being used to a certain limit.

B. Active Filters

To overcome drawbacks of passive filters active filters are

introduced. They inject harmonic voltage or current with

appropriate magnitudes and phase angle into the system and

cancel harmonics of non-linear loads. Active filters have the

advantage of being able to compensate for harmonic without

fundamental frequency reactive power concerns. This means

that the rating of the active power can be less than a

Paper ID: SUB152686 161





comparable passive filter for the same non-linear load and

the active filter will not introduce system resonances that can

move a harmonic problem from one frequency to another.

Active filter can be classified based on the connection

scheme as:

Shunt active filter

Series active filter

Hybrid active filter.

In this paper harmonic mitigation is done by using shunt

active power filter.

Figure 2: Active Power Filter

3. Shunt Active Power Filter (SAPF)

Shunt active power filter compensate current harmonics by

injecting equal but opposite harmonic compensating current.

In this case SAPF operates as a current source injecting the

harmonic components generated by the load but phase

shifted by 180º. This principle is applicable to any type of

load considered a harmonic source. Moreover, with an

appropriate control scheme, the active power filter can also

compensate the load power factor. The shunt active filter has

the capability of damping harmonic propagation between an

already-existing passive filter and the supply impedance. The

current compensation characteristic of SAPF is as shown.

Figure 3: Compensation Characteristics of SAPF

The compensation effectiveness of an active power filter

depends on its ability to flow with a minimum error and time

delay the reference signal calculated to compensate the

distorted load, current finally, the DC voltage control unit

must keep the total DC voltage constant and equals to a

given reference value. The DC voltage control is achieved by

adjusting the small amount of real power absorbed by the

inverter from the PCC. This small amount of real power is

adjusted by changing the amplitude of the fundamental

component of the reference current. The block diagram of a

shunt active power filter control scheme is shown and

consists of sensing the load currents and the point of

common coupling (PCC) voltages, reference current

generator, DC voltage control, injected current control and

the inverter. The Shunt Active Power Filter analyzed in this

paper is designed for 3-phase 4-wire systems and is capable

of compensating current harmonics, current unbalance and

power factor in 3-phase 4-wire electric systems.

3.1 Concept of P-Q Theory

The research work studied showed that a three phase four

wire system i.e. system having three phase with neutral

connection has large discrepancies when calculation of

neutral current takes place. This problem is identified and

can be solved using the concept of shunt active power filter.

It has further been seen that the neutral current concept offers

a large value when active shunt power filter has not been

deployed so validation for this also has to be done. Moreover

since the PI controller based system requires a hysteresis

control mechanism hence a hysteresis loop must also be

incorporated in the design to allow for more robust control

system.

An active rectifier based shunt compensator plays a vital role

in present-day static power compensation. This includes the

conventional compensation features like power factor

improvements, harmonic compensation and neutral current

elimination in a three-phase four- wire system. The

instantaneous power compensation theories have been

evolved essentially to execute the compensation or

correction in time domain.

Figure 4: Control Scheme of SAPF

The first instantaneous reactive power compensation theory,

popularly known as p-q theory was developed in Japan for a

three-phase three-wire system. The p-q theory was further

extended for a three-phase four wire system by defining zero

sequence power. The Instantaneous Reactive power theory

(IRP) p-q theory developed by Akagi, Kanazawa and Nabae

in 1983 uses time domain in order to define a set of

instantaneous powers. These Instantaneous powers are

defined in terms of instantaneous voltages and currents,






which are first, transformed from phase R, S and T to α, β, 0

coordinates by using the Clarke Transformation. This

transformation produces a stationary reference frame, where

coordinates α and β are orthogonal and the co-ordinate 0

corresponds to the zero sequence component. However, this

zero sequence coordinate differs from the zero sequence

components in the symmetrical component transformation.

3.2 Compensation with p-q Theory

This concept gives an effective method to compensate for the

instantaneous components of reactive power for three-phase

systems without energy storage. Instantaneous real and

imaginary powers have first been defined in the time domain.

The three phase voltages are sensed at the PCC and denoted

as ea, eb and ec. The resultant load side line currents are

sensed and denoted as iaL, ibL and icL. The Clarke

transformation for three phase voltages and line currents,

therefore are given as follows:

… (1)

… (2)

According to the p-q theory the set of instantaneous powers

in a three-phase system consists of the instantaneous zero

sequence power p0 defined as,

… (3)

Instantaneous real power p and the instantaneous imaginary

power q defined as,

..(4)

..(5)

These powers can also be written in a matrix form as,

..(6)

These quantities written in equation (3), (4) and (5) can be

elaborated as follows:

..(7)

..(8)

Where,

p0 - Mean value of the instantaneous zero-sequence power –

corresponds to the energy per time unity which is transferred

from the power supply to the load through the zero-sequence

components of voltage and current.

-Alternated value of the instantaneous zero-sequence

power – it means the energy per time unity that is exchanged

between the power supply and the load through the zero-

sequence components. The zero-sequence power only exists

in three-phase systems with neutral wire.

p - Mean value of the instantaneous real power – corresponds

to the energy per time unity which is transferred from the

power supply to the load.

- Alternated value of the instantaneous real power – It is

the energy per time unity that is exchanged between the

power supply and the load.

q - Instantaneous imaginary power – corresponds to the

power that is exchanged between the phases of the load. This

component does not imply any transference or exchange of

energy between the power supply and the load, but is

responsible for the existence of undesirable currents, which

circulate between the system phases.

This theory, unlike other theories does not only consider

each phase of the three phase system separately but also

defines them in terms of other phases. Moreover, this gives

us flexibility of using the theory for three-wire systems.

3.3 Instantaneous p-q theory for three-wire systems

Since, three-wire power systems do not contain zero

sequence current components, the zero sequence components

and in IRP p-q theory can be considered as zero. As a result,

the three-wire system can be represented in terms of reduced

vector Clarke coordinates. The reduced Clarke coordinates

are nothing but the representation of IRP p-q theory for

three-wire systems by neglecting the zero sequence

components. The reduced vectors for three phase Clarke

voltages and currents are determined as,

..(9)

The active and reactive power is written as:

..(10)

3.4 PI Controller

In this paper the proportional-integral (PI) controller is used

in SAPF. The control scheme comprises of PI controller,

limiter, and three phase sine wave generator for reference

current generation and generation of switching signals. The

peak value of reference currents is studied by regulating the

DC link voltage. The definite capacitor voltage will be

compared with a set reference value. The error signal is then

fed through a PI controller, which gives to zero steady error

in tracking the reference current signal. The schematic

representation of the control circuit is as shown:

Figure 5: Block representation of PI controller

The output of the PI controller is presumed as peak value of

the supply current (Imax), which is composed of two

components: (a) fundamental active power component of






load current, and (b) loss component of APF; to preserve the

average capacitor voltage to a constant value. Peak value of

the current (Imax) so found, will be multiplied by the unit

sine vectors in phase with the individual source voltages to

obtain the reference compensating currents. These expected

reference currents (Isa*, Isb*, Isc*) and detected actual

currents (Isa, Isb, Isc) are equated at a hysteresis band, which

delivers the error signal for the modulation technique. This

error signal chooses the operation of the converter switches.

In this current control circuit configuration the source/supply

currents Isabc are made to follow the sinusoidal reference

current Iabc, within a fixed hysteretic band. The width of

hysteresis window regulates the source current pattern, its

harmonic spectrum and the switching frequency of the

devices. The DC link capacitor voltage is always preserved

constant during the operation of the converter. In this

scheme, each phase of the converter is measured

independently. To increase the current of a particular phase,

the lower switch of the converter related with that particular

phase is turned on while to decrease the current the upper

switch of the corresponding converter phase is turned on.

4. Simulation Results

The simulation of the project was carried out in Matlab 8.1

and the project uses the simpower system library of

SIMULINK, the total harmonic distortion and fft analysis

was performed using power gui tool provided in sim power

systems, the results are as shown:

Figure 6: Result of THDi for the SIMULINK model

Figure 7: Graph of source current and THD without apf

Figure 8: Graph of neutral current and THD without apf

Figure 9: Graph of source current and THD in presence of

apf

Figure 10: Graph of neutral current and THD in presence of

apf

Figure 11: Graph showing PI controller operation






4.2 Comparison of Different Methodologies Adopted for

Harmonic Reduction

Parameters 1st 2nd 3rd 4th Our Work

SAPF

implement-

ation method

pq theory

with PI

contro-

ller

SRF

theory

pq theory pq theory

with MRC

pq theory

with PI

controller

Supply

system

Single

phase

Not

specified

Single

phase

3-phase3-

wire

3-phase 4-

wire

Load used Non

linear (6

IGBT

bridge)

Not

specified

Single

phase00d

iode

rectifier

Not

specified

Three phase

bridge

THDi 1.10% 1.01% 1.85% Not

specified

0.71%

Tools used PSCAD Simulink Simulink Simulink Simulink

5. Conclusion

In this paper an Active filter based on the instantaneous

active and reactive power component p-q method is studied.

Current harmonics consist of positive and negative sequence

including the fundamental current of negative sequence can

be compensated. Therefore, it acts as a harmonic and

unbalance current compensator. The analysis of IRP p-q

theory for non-sinusoidal conditions such as distorted supply

voltage and harmonic-generating loads also provides us with

an evaluation of performance of the p-q theory. For an

instance, it shows that the values of instantaneous powers in

a 3pN system with a balanced harmonic generating load (

HGL) supplied by a sinusoidal and symmetrical voltage does

not change with the harmonic order. In other words, the

values of instantaneous powers do not change when a 5th

order current harmonic generating load is replaced by a 7th

order current HGL. The total harmonic reduction for current

at the point of common coupling turns out to be 0.7% which

is way below the specification provided by IEEE 519

standard which specifies a nominal total harmonic distortion

to be under 5% , in this way the efforts put in to work are

aptly rewarding and the results were in compliance with

industrial standard.

6. Future Work

Experimental analysis can be done on Shunt Active Power

Filter based on instantaneous active and reactive power

component (p-q) method by developing prototype model in

the laboratory to verify the simulation result based on (p-q)

method with PI controller. It is important to develop this

system in laboratory because the field test of the theory

promises to give exciting results , although every care has

been taken to model the simulation as close as as possible to

the real world but the testimony of the project will remain

incomplete without the field testing of the model.

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p-q power Theory for compensating nonsinusoidal

systems," Nonsinusoidal Currents and Compensation,

2008. ISNCC 2008. International School on , vol., no.,

pp.1-10, 10-13 June 2008.

[2] Akagi, H. and Nabae, A. (1993), The p-q theory in

three-phase systems under non-sinusoidal conditions.

European Transactions on Electrical Power, 3: 27–31.

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Compensation

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